1. Field of the Invention
The present invention relates generally to global navigation satellite systems (GNSS) receiver technology, and in particular to an application-specific integrated circuit (ASIC) for down-converting dual frequency signals from a GNSS frequency superband simultaneously.
2. Description of the Related Art
Global navigation satellite systems (GNSS) include the Global Positioning System (GPS), which was established by the United States government and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in three frequency bands, centered at 1575.42 MHz, 1227.60 MHz and 1176.45 MHz, denoted as L1, L2 and L5 respectively. All GNSS signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error. Different levels of accuracies can be achieved depending on the techniques employed.
GNSS also includes Galileo (Europe), the GLObal NAvigation Satellite System (GLONASS, Russia), Beidou (China), Compass (proposed), the Indian Regional Navigational Satellite System (IRNSS) and QZSS (Japan, proposed). Galileo will transmit signals centered at 1575.42 MHz, denoted L1 or E1, 1176.45 denoted E5a, 1207.14 MHz, denoted E5b, 1191.795 MHz, denoted E5 and 1278.75 MHz, denoted E6. GLONASS transmits groups of FDM signals centered approximately at 1602 MHz and 1246 MHz, denoted GL1 and GL2 respectively. QZSS will transmit signals centered at L1, L2, L5 and E6. Groups of GNSS signals are herein grouped into “superbands”.
To gain a better understanding of the accuracy levels achievable by using GNSS, it is necessary to understand the types of signals available from the GNSS satellites. One type of signal includes both the coarse acquisition (C/A) code, which modulates the L1 radio signal, and the precision (P) code, which modulates both the L1 and L2 radio signals. These are pseudorandom digital codes that provide a known pattern that can be compared to the receiver's version of that pattern. By measuring the time-shift required to align the pseudorandom digital codes, the GNSS receiver is able to compute an unambiguous pseudo-range to the satellite. Both the C/A and P codes have a relatively long “wavelength,” of about 300 meters (1 microsecond) and 30 meters ( 1/10 microsecond), respectively. Consequently, use of the C/A code and the P code yield position data only at a relatively coarse level of resolution.
The second type of signal utilized for position determination is the carrier signal. The term “carrier,” as used herein, refers to the dominant spectral component which remains in the radio signal after the spectral content caused by the modulated pseudorandom digital codes (C/A and P) is removed. The L1 and L2 carrier signals have wavelengths of about 19 and 24 centimeters, respectively. The GNSS receiver is able to “track” these carrier signals, and in doing so, make measurements of the carrier phase to a small fraction of a complete wavelength, permitting range measurement to an accuracy of less than a centimeter.
In stand-alone GNSS systems that determine a receiver's position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the satellite's clock reference, the location of the orbiting satellite, ionospheric-induced propagation delay errors, and tropospheric refraction errors. A more detailed discussion of these sources of error is provided in U.S. Pat. No. 5,828,336 by Yunck, et al.
To overcome the errors of stand-alone GNSS, many kinematic positioning applications make use of multiple GNSS receivers. A reference receiver located at a reference site having known coordinates receives the satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and at the remote location, these errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates. The technique of differencing signals is known in the art as differential GNSS (DGNSS). The combination of DGNSS with precise measurements of carrier phase leads to position accuracies of less than one centimeter root-mean-squared (centimeter-level positioning). When DGNSS positioning utilizing carrier phase is done in real-time while the remote receiver is potentially in motion, it is often referred to as Real-Time Kinematic (RTK) positioning.
One of the difficulties in performing RTK positioning using carrier signals is the existence of an inherent ambiguity that arises because each cycle of the carrier signal looks exactly alike. Therefore, a range measurement based upon carrier phase has an ambiguity equivalent to an integral number of carrier signal wavelengths. Various techniques are used to resolve the ambiguity, often with some form of double-differencing. The prior art related to this includes U.S. Pat. No. 4,170,776 by MacDoran, U.S. Pat. No. 4,667,203 by Counselman, U.S. Pat. No. 4,963,889 by Hatch, U.S. Pat. No. 5,296,861 by Knight, and U.S. Pat. No. 5,519,620 by Talbot et al. Once ambiguities are solved, however, the receiver continues to apply a constant ambiguity correction to a carrier measurement until loss of lock on that carrier signal. Regardless of the technique employed, the problem of solving integer ambiguities, in real-time, is always faster and more robust if there are more measurements upon which to discriminate the true integer ambiguities. Robust means that there is less chance of choosing an incorrect set of ambiguities. The degree to which the carrier measurements collectively agree to a common location of the GNSS receiver is used as a discriminator in choosing the correct set of ambiguities. The more carrier phase measurements that are available, the more likely it is that the best measure of agreement will correspond to the true (relative to the reference GNSS) position of the remote GNSS receiver.
One method, which effectively gives more measurements, is to use dual frequency (DF) receivers for tracking delta-range measurements from P code modulation on the L1 and L2 carriers simultaneously with the L1 C/A code generating code phase measurements. The L1 and L2 carriers are modulated with codes that leave the GNSS satellite at the same time. Since the ionosphere produces different delays for radio carriers of different frequencies, such dual frequency receivers can be used to obtain real-time measurements of ionospheric delays at various receiver positions. The L1 and L2 ranging measurements are combined to create a new L1 ranging measurement that has an ionospheric delay of the same sign as the ionosphere delay in the L1 pseudorange. Accurate ionospheric delay information, when used in a position solution, can help produce more accuracy. Absent such real-time ionospheric delay measurements, other correction techniques are commonly used, such as differential GNSS (DGNSS), proprietary third party satellite augmentation system (SAS) services available on a paid subscription basis, or the U.S.-sponsored Wide Area Augmentation System (WAAS).
As compared to single-frequency (typically L1) receiver systems, previous dual-frequency receiver systems have tended to be relatively expensive because of their additional components for accommodating L2 measurements. Moreover, the additional components tended to consume more power and required additional space. Still further, dual-frequency receivers should be adaptable for use with all present and projected GNSS, transmitting signals which can be grouped into two “superbands” of radio signal frequencies generally in the range of about 1160 MHz to 1250 MHz and 1525 MHz to 1613 MHz. Accordingly, a preferred multi-frequency receiver should be: a single, application-specific integrated circuit (ASIC); programmable for down converting various pairs of frequencies; minimally-sized; and capable of operating with minimal power.